1. Field of the Invention
The present invention relates to an apparatus for grinding semiconductor wafers, and more particularly to means for keeping a semiconductor wafer, which is fixedly held on the wafer-gripping surface, parallel to the grinding surface of the grinding means in the semiconductor wafer grinding apparatus.
2. Description of Related Art
FIG. 8 shows a conventional grinding apparatus 100 having a semiconductor wafer holder 117 and a grinding means 107 set up on its L-shaped stand. A screw rod 102 is rotatably fixed to the rear side of the vertical wall 101 of the L-shaped stand, extending parallel to the vertical wall 101. The screw rod 102 is driven by an associated power drive 103 so that its movable part 104 may be moved up and down. The movable part 104 has a slide 105 integrally connected thereto for moving on vertical rails 106 laid on the front side of the vertical wall 101. The slide 105 carries grinding means 107. The vertical position of the movable part 104 can be determined by a linear scale 108, which is fixed to the rear side of the vertical wall 101.
The grinding means 107 comprises a spindle 109 and a spindle housing 110 for rotatably holding the spindle 109, which has a grindstone mount 111 on its end. The grindstone mount 111 has a grinding wheel 113 fixed to its bottom, and a grindstone 112 is attached to the grinding wheel 113, which can be rotated by the spindle 109.
The rotary holder 117 is positioned on the base 114 of the L-shaped stand. It can be rotated by an associated servomotor 116, which has an encoder 115 equipped therewith. The wafer-gripping surface 118 of the rotary holder 117 can hold a semiconductor wafer by applying a negative pressure to the semiconductor wafer. The rotary holder 117 is seated on a support member 119, which has three level adjusting screws 120 equal angular distance apart from each other, thereby permitting the wafer-gripping surface 118 to be set parallel to the grindstone 112.
In operation a selected semiconductor wafer is put on the wafer-gripping surface, which is sucked and fixedly held thereon. The spindle 109 is put in rotation, and at the same time, the grinding means 107 is lowered. The spindle 109 is rotated at high speeds, and accordingly the grinding wheel 113 is rotated at high speeds. The rotating grindstone 112 is pushed against the semiconductor wafer to make its surface smooth by rubbing on the hard grindstone 112.
In grinding it is required that the grinding accuracy be increased to the extent that the wafer has an even thickness over its whole area with keeping the total thickness variation (TTV) constant as possible. To meet this requirement it is necessary that the wafer-gripping surface 118 is put in exact parallelism relative to the undersurface of the grindstone 112.
The rotary holder 117 and the grinding means 107 are set up very carefully to assure the strict parallelism as required therebetween. No matter how carefully these parts may be set up, however, a minimum misalignment in the order of several microns cannot be reduced. To reduce such a minimum misalignment the three leveling screws 120 are used in the grinding apparatus 100.
As a recent tendency the degree of integration in semiconductor devices has been increasing, and the semiconductor wafer size has been increasing, too. Accordingly it is required that the permissible minimum misalignment be reduced to the order of nanometer. The leveling screws 120, however, cannot meet such requirement.